The invention is directed to a process for the hydrogenation of unsaturated triglycerides, such as edible oils, to produce partially saturated triglycerides (oils/fats), as well as to hydrogenated edible oils obtainable by such as process.
It is known to prepare partially hydrogenated triglycerides, especially hardened fats for use in cooking and frying fat, bread spread, such as margarine, and products having a lower fat content, or frying oils and lubricants from triglyceride, i.e. vegetable oil, such as soybean oil or rapeseed oil, by catalytic hydrogenation in the presence of hydrogen. This hydrogenation is necessary, among other reasons, to increase the oxidation stability (decrease of the amount of linolenic acid) and to obtain the desired melting behavior of the triglyceride, for instance for the purpose of obtaining sufficient spreadability. Hydrogenation can take place utilizing conventional hydrogenation catalysts, such as nickel or precious metal catalysts. The triglycerides used herein are poly-unsaturated, mainly based on C12 to C22 fatty acid moieties. The majority of the fatty acid moieties is formed by the C16 and C18 fatty acids The hydrogenation generally results in mono- or di-unsaturated fatty acid moieties in the triglyceride.
As consumer awareness of the health hazards of the use of products obtained by hydrogenation grows, so grows the desire to reduce as far as possible the content of trans-isomers in the unsaturated fatty acids. In the natural products, the cis-isomer occurs predominantly. Besides hydrogenation also isomerization usually occurs, resulting in the formation of trans-isomers. In the conventional catalytic hydrogenation of soybean oil to form a product having a content of completely saturated fatty acids of from about 12 to 14% (iodine value of about 70), in addition to the amount naturally present therein (generally about 15 wt. %) an increase of the trans-isomer content of about 30-50% is obtained. Typical reaction conditions herein comprise the use of a conventional nickel hydrogenation catalyst, a temperature of between 175 and 200° C. and 0.7 to 2 bar hydrogen pressure. In the non-food applications of partially hydrogenated triglycerides, such as lubricants, the presence of trans-isomers is less preferred because of the increased melting point of trans-isomers. It is to be noted that some pre-treatments of poly)unsaturated triglycerides, such as cleaning or decoloration, may lead to a small degree of isomerization. This usually accounts for the presence of about 0.5 to 2 wt. % of trans-isomers in the triglyceride prior to being subjected to hydrogenation.
The method for preparing partially saturated fatty acid-triglycerides with a low trans-isomer content has already been investigated extensively. One approach consists in adjusting the hydrogenation conditions, whereby hydrogenation is promoted in relation to isomerization by the use of much hydrogen at the surface of the catalyst. This means that it is required to work at a low temperature, at a high hydrogen partial pressure and with a proportionally slight amount of catalyst in relation to the amount of component to be hydrogenated. With this method it is possible to lower the trans-isomer content to about 10%, the saturated fatty acid content increase being about 15%. However, this method is commercially little attractive, because major capital investments would have to be made to achieve higher pressures.
It is also known to use supported precious metal catalysts for the hydrogenation of triglycerides. These catalysts have the property that they produce too much fully saturated fatty acid moieties. A review of the problems and possibilities of reducing trans-isomer formation is given in “Hydrogenation of oils at reduced TFA content” in Oils & Fats International, July 2004 (pages 33-35).
It would be very attractive to have a precious metal catalyzed hydrogenation process for unsaturated triglycerides, which process produces hydrogenated triglycerides having reduced amounts of trans-isomers and saturates, compared to conventional hydrogenation processes.